It is known in the art that fruit juices such as citrus juices can by means of an ion exchange treatment be made to undergo a change in the perceptive acidity of such juices. Taste tests have established that the natural acidity of such juices is objectionably strong to a significant number of prospective purchasers who would prefer a less tart or sharp flavor. A process to achieve this result was first disclosed by Kilburn et al in U.S. Pat. No. 3,165,415, filed Oct. 4, 1960 and issued Jan. 12, 1965, wherein the juice to be treated was passed through a multi-compartment electrodialysis system in alteration compartmentwise with a liquid electrolyte, the compartments being separated by an anion selective membrane permeable to passage therethrough of ions extracted from the juice and the electrolyte to thereby substitute one for the other and consequently alter the pH of the juice being treated. The process was described as equally useful for inducing an increase in the pH of a pH deficient juice, such as tomato juice, as to decrease the pH of an excessively acidic juice, such as orange or grapefruit juice, the ion permeable membrane being in the one case a film of a cation exchange resin and in the other case a film of an anionic exchange resin. The basis of this discovery was that, although the pH of natural fruit juices changes during their natural production season, the concentration of salts of weak acids which such juices contain remains substantially constant throughout such season. Hence, although the theoretical pH of any buffered solution, i.e., containing a free acid and its salts, depends upon the relative concentrations of both of the free acid and the salt, if the salt concentration remains constant, it follows that overall pH variation must in the case of citric acids be due to the free acid content of such juices. This creates the possibility for extracting proportions of the free acid content of citrus and like juices by means of anion exchange or alternatively to convert citric salts to free acid by a cation exchange resin to increase the acidity for other low acid juices.
The taste perception of acidity in the human mouth is not determined entirely by the pH of the substance being tasted, as Kilburn et al carefully explain, but is influenced by other factors, notably sweetness, as well as saltiness and possibly psychological factors. Within the citrus industry, the convention has been adopted of expressing tartness in terms of a ratio of the Brix value of the juice, which is expressed as the number of grams in the juice of soluble solids, mainly consisting of soluble sugars, to the acid concentration, expressed as grams of anhydrous citric acid, per 100 ml of juice. The correlation between this ratio and the perceived tartness of the fruit is now well accepted by the industry and is employed, among other things as a definition for the maturity of the fruit in regulations governing the sale of various types of citrus fruits and juices.
As indicated, Kilburn et al employ as the means for effecting an interchange of ions to and from the juice and electrolyte, an ion permeable membrane in the form of a synthetic film of ion exchange resin, separating alternating juice and liquid electrolyte chambers passed respectively by concurrent flows of juice and liquid electrolyte. A caustic electrolyte, e.g. sodium hydroxide, results in an exchange of hydroxyl ions from the electrolyte for the citrate ions of the juice while an acidic electrolyte results in an exchange of hydrogen ions for cations, e.g., potassium ions, from the citrate salts of the juice.
Kilburn et al acknowledge (column 5, lines 65 through column 6, line 10) their deliberate choice of ion exchange resin films in contrast to the bead form usual for other conventional ion exchange processes but justify their choice on the grounds of the unsuitability of bead form resins for treating the juices in question. Thus, citrus juice typically contains a substantial amount of suspended pulp matter of widely varying particle size and if the juice flow were to be directed downwardly through an ion exchange resin bed in the usual manner, the suspended pulp solids would soon clog the bed to the point of inhibiting further flow therethrough, necessitating an interruption for cleaning.
Further, Kilburn et al judge the regeneration stage, which is an inherent part of conventional bed processes, to be unsuited for citrus juice treatment, leading to contamination and dilution of the juice by the regeneration and/or rinsing liquids. For these reasons, Kilburn et al conclude "Thus, ion exchange resin in bead form is not suitable for continuous operation as required for commercial application."
About a decade later, research on the production of a reduced acid orange juice was spearheaded by the Coca Cola Company, Foods Division, and in the Fall of 1979, a paper was presented on behalf of that group by Dr. K. Assar, entitled "Reduced Acid FCOJ" to the Florida citrus industry, which paper was subsequently published in the "Proceedings of the 19th Annual Short Course for the Food Industry at the Institute of Food and Agricultural Sciences of the University of Florida, Gainesville, Fla." during 1980. This paper describes the efforts of this group to produce and market a reduced acid frozen concentrated orange juice having the approval of both Florida and federal authorities which were eventually successful and led to the promulgation by both the Florida state authorities and the FDA of an approved definition for such a juice, the federal regulation appearing in CFR, Title 21--Food and Drugs--.sctn..sctn.146.150 and 173.25. The research summarized here focused on the acceptability of the reduced acid product from various health aspects, confirming the initial conclusion of Kilburn et al that the ion exchange treatment had no significant consequences on the vitamin content of the treated juice and extending that conclusion to other nutrient components, such as ascorbic and folic acids, minerals and amino acids.
The treatment procedure as described in this paper contemplated the passage of either freshly extracted juice or a diluted form of previously obtained bulk concentrate downwardly through a bed or column of ion exchange resin until the pH of the effluent juice passed below pH 4.6. Either juice was centrifuged to reduce its pulp content before the exchange step, it being noted that removal of some of the pulp content served to inhibit development of high back pressures in the column due to plugging of the resin bed with pulp particles. The possibility of re-introducing added pulp during subsequent processing was mentioned.
The utilization of ion exchange in conjunction with liquid food products other than fruit juices was apparently the subject of research much earlier than in the citrus industry, one important field being in the treatment of milk products. Thus, in U.S. Pat. No. 2,233,178, issued Feb. 25, 1941, there is disclosed for purposes of producing an ice cream high in nonfat milk solids but free of a perceptible grittiness or sandiness in the mouth due to crystallization of lactos therein by treating either whole milk, skim milk, and the like containing nonfat milk solids with or without prior acidification, with an active base exchange material. It was said that the process could be carried out either batchwise or continuously in several different ways. On the one hand, the milk product could be agitated in contact with the finely divided base exchange resin, as in a churn, or the liquid product could be passed either downwardly or upwardly through a bed of ion exchange material maintained if desired under agitation, and prior heating of the milk product was an optional feature to facilitate its passage through the bed.
In U.S. Pat. Nos. 2,465,906 and 2,465,907 the treatment with an ion exchange resin of a different kind of milk product is disclosed; namely, whey. As is known in the art, whey is the milk serum which remains after substantially complete removal of the fat and casein contents of the original milk and consists essentially of an aqueous solution of the milk sugar lactose. More specifically, according to Modern Dairy Products, Chemical Publishing Co., Inc., New York, 1970, page 187, a typical whey resulting after precipitation with rennet of the milk fat and casein, contains less than 1% milk protein, about 1/2% milk fat, about 5% lactose, 1/5% lactic acid and 1/2% ash, and in excess of 93% water. It differs compositionwise from plain skim milk (page 186) in its reduced milk protein content compared to 3.7% for plain skim milk.
According to the common disclosure of these two patents, the whey is treated to remove a ma3or part of its mineral and acidic (amino acids) content so as to avoid the creation of deposits of calcium phosphate and lactose during subsequent heat treatment. For this purpose, the whey liquid is first contacted with a decationizing medium, i.e, a cation exchange medium to replace its metallic ions with hydrogen atoms with a consequential reduction in its pH value, and then contacted with a de-acidifying medium, i.e., an ion exchange medium, to increase its pH value to about neutrality. In both stages, preference is expressed for directing the whey liquid upwardly through a bed of the particles, in the first stage "in order to disperse said medium and thereby prevent entrapment of particulate matter" (col. 3, 11. 50-53) and the second "in order to flush away precipitate of protein material into a zone of higher pH conditions where the protein is re-dissolved" (col. 3, 11. 69-72).
The most common use of ion exchange resins is in the conditioning of water and the standard procedure in this field is the percolation of the water to be treated downwardly through a fixed bed of the ion exchange resin particles during all active treatment stages of exchange and regeneration. However, upflow has been suggested for the stage of regeneration of the exhausted resin in order that the most active region of the regenerated resin bed would be the last region contacted by the water being treated. When practiced during regeneration, liquid upflow has been carried out with such force or flow velocity as to cause substantial complete inversion of the resin bed, forcing the resin bed upwardly into a fixed condition against an upper retaining member so as to create a bed-liquid relationship in effect the same as existed during downflow, it being recognized that continued compaction of the bed was necessary to avoid channeling. Positive bed compaction means to give added assurance against channeling is shown in U.S. Pat. No. 3,180,825.
In conventional ion exchange methods, after the resin bed has been filled, i.e., soiled with extraneous impurities and any fines from the bed particles, standard operating practice is to subject the bed to upflow for backwashing purposes with a quantity of wash water sufficient to cause an increase or expansion in the volume of the bed, thereby increasing the void or open space volume between the resin particles and inducing a partial fluidization of the bed, such upflow being continued for a sufficient period to flush out all of the undesired matter from the bed. Obviously, however, this backwashing practice is purely a cleansing step unreleated to any ion exchange as such.